The invention relates to a letterbox television system, i.e. a television system in which a wide aspect ratio picture is transmitted between two horizontal bars so as to form a standard aspect ratio picture. In such a television system, vertical resolution enhancement signals are transmitted to improve the vertical resolution upon display on an enhanced wide aspect ratio television receiver, while these signals are transmitted in such a way that they are as invisible as possible to the viewer of a current standard aspect ratio television receiver. More specifically, the invention relates to the manner of transmitting the vertical enhancement signals.
EP-A-0,357,144 discloses a letterbox television system of the type described in the opening paragraph. The vertical resolution enhancement signals are low-pass filtered up to 1.25 MHz because only a low-pass portion of the enhancement signals may be modulated onto the quadrature sideband of the picture carrier. The remaining high-frequency part above 1.25 MHz of the vertical resolution enhancement signals is transmitted in the horizontal bars. Modulation of the quadrature sideband of the picture carrier has a number of drawbacks. First, it is a rather complex technique. Secondly, modulation of the quadrature sideband of the picture carrier disturbs the phase of the picture carrier so that the automatic frequency control loop in existing receivers will probably be disturbed. Thirdly, when intercarrier sound decoding is used, the phase deviations caused by the quadrature modulation of the picture carder are likely to render sound demodulation problematic.
To solve these problems, EP-A-0,460,750 discloses a letterbox television system in which the vertical resolution enhancement signals are only transmitted during the horizontal bars. The vertical resolution enhancement signals are modulated onto a subcarrier to reduce any low-frequency components which would be too visible upon display on a current standard aspect ratio television receiver, and to mitigate compatibility problems with synchronizing circuits in existing transmitters and receivers. Unfortunately, the fact that the visible wide aspect ratio part of the letterbox television signal is transmitted directly while the vertical resolution enhancement signals are transmitted in a modulated form, causes phase differences between, for example, a 3 MHz component which is transmitted in the visible wide aspect ratio part of the letterbox television signal on the one hand, and on the other hand a 3 MHz component which is transmitted in the horizontal bars at a frequency of 4.43-3=1.43 MHz after modulation onto a 4.43 MHz subcarrier.